Turbine blade and method of making same



Nov. 14, 1961 E. SWENSON TURBINE BLADE AND METHOD OF MAKING SAME FiledJan. 26, 1959 and ATTORNEY United States Patent 3,008,855 TURBINE BLADEAND METHOD OF MAKING SANIE Ernest L. Swenson, Bedford, Ind., assignor toGeneral Motors Corporation, Detroit, Mich., a corporation of DelawareFiled Jan. 26, 1959, Ser. No. 788,944 3 Claims. (Cl. 14832) Thisinvention relates to turbine blades, compressor blades or the likehaving improved thermal shock resistance and to a method formanufacturing the same.

In turbojet engines a turbine operated by burning gases drives a blowerwhich furnishes air to the burner. Such turbines operate at a relativelyhigh temperature and the capacity of such engines is limited by theability of the turbine blades to withstand the high temperaturesinvolved. An important factor is the ability of the blades to withstandthe thermal shock involved in the heating and cooling of the blades inthe operation of such engines. It has been found that blade failures dueto thermal shock are associated with the crystal or grain boundary ofthe metal of which the blades are formed.

It is an object of this invention to provide metal articles havingimproved thermal shock resistance and more particularly to provideturbine blades, compressor blades and like articles having improvedthermal shock resistance. A further object of the invention is toprovide turbine blades or the like formed of high temperature nickel orcobalt base alloys having airfoils of which at least the leading edgethereof consists of substantially a single crystal. 'It is anotherobject of this invention to provide a method for forming a turbine bladeor the like having improved thermal shock resistance, and preferably ablade or the like of which at least the leading edge thereof consists ofa single crystal.

These and other objects are accomplished by forming a blade of a hightemperature nickel or cobalt base alloy containing both chromium andcarbon in a process wherein the alloy is melted and cast under vacuum,the alloy melt being held under vacuum for a prolonged period before thecasting thereof, and preferably for a time sufficient to provide a castblade having at least the leading edge thereof formed substantially of asingle crystal. The resulting blade has markedly superior thermal shockresistance as compared to a blade cast of the same alloy utilizingconventional casting procedures Other objects and advantages will morefully appear from the following detailed description of the invention inconjunction with the accompanying drawing in which:

FIGURE 1 is a schematic illustration of apparatus suitable for carryingout the method of the present invention.

FIGURE 2 is an enlarged photolithic view of a macroetched blade whichhas been vacuum cast by conventional procedures.

FIGURE 3 is an enlarged photolithic view of a macroetched blade cast inaccordance with the process of the present invention.

As above-indicated, the present invention is concerned with themanufacture of turbine blades or like articles having markedly improvedresistance to thermal shock and with a process of manufacture whereinthe article or blade is cast from a nickel or cobalt base alloycontaining both chromium and carbon and wherein the melting of the alloyis performed under a high vacuum and the melt is maintained under suchvacuum for a prolonged period of time, and preferably for a timesufficient so that when the article or blade is cast while still undervacuum, a blade is formed which has at least the leading edge thereofconsisting substantially of a single grain or crystal.

Suitable high temperature alloys for use in the present inventioninclude an alloy of the type set forth in the Callaway et al. Patent2,688,536, assigned to the assignee of the present invention andconsisting essentially as follows:

0.10% to 0.20% carbon 0.25% max. manganese 0.75% max. silicon 13% to 17%chromium 4% to 6% molybdenum 1.50% to 3.00% titanium 2.00% to 4.00%aluminum 0.01% to 0.10% boron 8% to 12% iron Balance nickel Anotheralloy suitable for use in the present invention consists of the abovealloy in which the iron content is reduced from about 0.5% to 8.00% andpreferably from about 3.5% to 6.0%.

Other suitable high temperature nickel base alloys and cobalt basealloys containing both chromium and carbon, well known in the art, areas follows:

A-286 Hastelloy R Hastelloy C 25.00% nickel. 65.0% nickel. 55.00%nickel. 141.75% chromium. 15.5% chromium. 16.00% chromium. 0.08% carbon.1.50% cobalt. 0.15% carbon.

1.25% molybdenum. 0.10% carbon. 17.0% molybdenum. 1.90% titanium. 5.00%molybdenum. 5.00% tungsten. 0.35% aluminum. 2.50% titanium. 1.00%aluminum. 0.25% vanadium. 2.25% aluminum. 5.50% iron. Balance iron.7.00% iron.

Waspalloy Hastelloy 06 8-497 13.5% cobalt.

19.5% chromium. 0.10% carbon.

4.25% molybdenum. 2.50% titanium. 1.50% aluminum. 2.00% :iron.

20.0% cobalt.

20.0% chromium. 20.0% nickel.

0.10% carbon. 2.00% tungsten. 8.00% molybdenum. Balance iron.

19.0% cobalt. 14.0% chromium. 19.5% nickel.

0.42% carbon.

4.00% tungsten. 1.00% molybdenum.

; 4.00% columbinm.

Balance nickel. Balance iron.

Suitable apparatus for carrying out the invention is shown in FIGURE l1which includes an air-tight chamber 10 which may be evacuated throughthe conduit 12 and vented to the atmosphere through the conduit 14. Aclosure 16 is provided on the chamber 10 to permit access thereto.Tiltahly mounted within the chamber 10 is an induction furnace 18containing therein a crucible or suitable receptacle .20 for holdingmolten metal.

Below the vacuum chamber 10 is a second vacuum chamber 22 incommunication with the chamber 10 through the opening 24 when theclosure 26, shown in a partially open position, is in an open orvertical position. The second chamber 2 2 is also provided with a vacuumconduit 28, a venting conduit 30 and an access door 32 shown in a closedposition. Within the chamber 22 is a vertically movable platform 34mounted on a vertical shaft 36. Means is provided (not shown) for movingthe shaft 36 vertically and thereby moving the platform 34 from itslowered position as shown in the drawing to a raised position within thechamber 10.. The shaft 36 is provided with a suitable sealingarrangement whereby the chamber 22 may be maintained in a highlyevacuated condition.

In general the operation of the above-described apparatus is as follows.The access door 16 'is opened and the crucible 20 is charged with theconstituents of the alfloy to be used in the process of the invention.Thereafter the closures 16 and 26 are closed, and the chamber 10 isevacuated to a high vacuum of about 20 microns of mercury or less. Next,the induction furnace 18 is placed into operation to melt the metalwithin the crucible 20. The alloy melt is then maintained under vacuumfor a predetermined time as will be hereinafter more fully described.

Meanwhile a suitable mold 38 in which a turbine blade or the like is tobe cast is placed on the platform 34 through the access door 32. Thechamber 22 is then evacuated to approximately the same degree as chamber10. The interconnecting door 26 is then opened, the mold 38 is raisedinto chamber and the molten metal within the crucible 20 is poured intothe mold. Thereafter the mold 38 is lowered into the chamber 32, theinterconnecting door 26 is closed, the chamber 22 is vented and the mold338 is removed. The apparatus is, of course, provided with suitableremote control means and suitable Windows to enable the above operationsto be performed without breaking vacuum within the chambers in thecourse of operating the apparatus.

As indicated above, the method of the present invention involves meltingand casting a nickel or cobalt alloy containing both chromium and carbonunder vacuum.. It has been found that holding the alloy melt under highvac uum for a prolonged period of time before casting provides the alloywith an improved high temperature duetility and further results incasting having remarkably larger crystals. By holding the melt undervacuum for a suflicient time after the melting thereof, a time in theorder of one hour, a turbine blade or the like may be cast wherein theleading and trailing edges thereof consist of substantially a singlecrystal. The thermal shock resistance of such a blade has been found tobe remarkably improved.

The above-described phenomenon is not clearly understood, however,experimental evidence indicates that the increased grain size is afunction of the time that the melt is held under vacuum and the chemicalcomposition of the alloy. Grain size has been found to increase with theincreased time that the melt is held under vacuum before casting. Thisincrease in grain size with increased time under vacuum is accompaniedby the decrease in the percentage of both the chromium and carbon in thealloy. It is believed that nucleating centers for crystal formationcontained in the carbon and chromium become more dispersed or decreasedby the vacuum application and that these centers are containedessentially in the chromium and carbon. It has further been found thatchromium and carbon added to the vacuum melt will again result inrelatively fine grain structure on casting, and after such additions ofchromium and carbon, it is necessary to again hold the melt under vacuumfor a prolonged period of time to achieve a coarsening of grainstructure.

The invention is illustrated specifically by melting an alloy undervacuum, holding the alloy under vacuum for successively greater periodsof time, and then casting a turbine blade after each period while themolten metal is still under vacuum. The following table gives thechemical analysis of each casting and the time for which the It will benoted that the percentage composition of chromium and carbon in eachcasting decreases with time under vacuum. FIGURE ZilIustrates themacro-etched crystalline structure of casting No. 1, cast under vacuumafter holding the melt under vacuum of about 20 microns of mercury forabout 30 minutes. In conducting the above experiment, approximately 25minutes were required to melt the alloy and to bring it to a pouringtemperature of about 2650" F. and an additional 5 minutes were requiredto cast each mold. Accordingly, casting N0. 1, cast after 30 minutesunder vacuum, represents a casting formed as soon as feasible, after thevacuum melting step has been accomplished. It will be noted that theleading edge 40 of the blade shown in FIGURE 2 consists of a relativelylarge number of crystals. This blade failed after approximately 800thermal shock cycles. In each cycle the leading edge of the blade wasflameheated to about 800 F. and air-cooled to about 500 F.

FIGURE 3 illustrates the macro-etched crystalline structure of thecasting No. 10, cast under vacuum while holding the melt undervacuum'for a period of about 96 minutes. As shown in FIGURE 3, theleading edge 42 of this blade consists of a single crystal. This bladedid not fail after being subjected to 1500 thermal shock cycles of thetype above-indicated. The macro-etched castings 2 to 9 (not shown)indicated a progressively coarser crystalline structure, but the leadingedge of each of these blades consisted of a plurality of crystals. Thesecastings also evinced a progressively greater resistance to thermalshock. Markedly improved thermal shock resistance resulted in bladeswhich were cast of metal held under Vacuum for at least 15 minutes afterthe melting of the alloy, and markedly superior results were obtainedwhen holding the melt under vacuum about 30 minutes before casting.Thus, casting No. 2 also failed after about 800 thermal shock cycleswhereas casting No. 3 failed after 900 thermal shock cycles and castingsNos. 4, 6 and 8 withstood 1000 thermal shock cycles. It is readilyapparent from the above data that a prolonged vacuum treatment aftermelting in the order of one hour produces castings having greatlysuperior thermal shock resistance and that an optimum blade results whenat least the leading edge of the blade consists of a single crystal.

Since the above-described process involves the removal of chromium andcarbon from the melt, it is necessary to periodically add chromium andcarbon to the alloy melt to maintain the chromium and carbon content inthe casting within the desired specifications.

Apart from the vacuum aspects of the above-described process, more orless conventional investment molding procedures are preferably employed.In forming the mold 38 a pattern of wax or other suitable lowtemperature, heat-destructible material such as polystyrene or resinous,polymerized derivatives of acrylic or methacrylic acid is formed insuitable injection mold apparatus or by other suitable means.

The surfaces of the pattern are next coated with a ceramic wash orcoating material which is to provide the smooth casting surface on therefractory mold to be melt was was held under vacuum. formed. Thiscoating material comprises an aqueous Time 7 Heat Ostg. No. Gr Fe O SiMn Ti Al Mo B Held Under Vacuum OriginalAnalysis. 15.13 4.90 15 .1100.10 2. 55 3. 53 5.85 .054 1 15. 50 5. 50 15 .015 005 2. 45 3. 87 4.90035 30 5. 45 15 005 2. 50 3. 93 4. 90 039 35 5.50 15 005 2.45 3.85 4.90034 43 5. 50 14 004 2. 50 3. s5 4. 90 037 53 5. 40 14 003 2. 5s 3. 95 4.37 034 58 5. 45 13 124 003 2. 47 3. 90 4. 55 075 55 5. 40 13 152 003 2.50 3. 00 4. 35 039 70 5. 40 12 003 2. 55 3. 97 4. 37 090 35 5. 40 12 150003 2. 68 4. 10 4. 55 037 90 5. 55 10 154 003 2. 52 4. 00 4. 37 085 95dispersion of conventional finely comminuted refractory materials, abinder, such as an air-setting silicate cement, and defoaming andWetting agents.

Coating of the pattern with the ceramic Wash is preferably accomplishedby dipping the pattern in the coating solution. Although in someinstances the ceramic coating may also be applied by spraying orpainting it on the pattern or in any other suitable manner, dipping ispreferred because it assures more uniform coating of all the patternsurfaces and is the simplest method of application.

The clip coat slurry is preferably kept in constant motion by stirringmeans except during the actual dipping operation. However, the mixingaction should not be such as to unnecessarily introduce air into theslurry, and care should be exercised in immersing the pattern in theslurry to prevent air entrapment on the pattern. Normally the dip coatsolution is retained at room temperature during the dipping operationbecause excessive heat can result in distortion of the plastic or waxpattern. The excess coating material is permitted to drain off prior tosubsequent treatment and investment.

After the pattern has been completely coated with the dip coat slurry,it may be sanded or stuccoed to provide a rough surface on the coating,thus insuring greater adhesion between the principal refractory portionof the mold and the dip coat on the pattern. This sanding may beaccomplished by merely screening or otherwise applying silica sand orother suitable refractory materials in known manner to the outer coatedsurface of the destructible pattern. When all the molding surfaces ofthe pattern have been effectively covered with sand, the pattern shouldbe air dried.

Thereafter the pattern is invested in a mold containing a relativelycoarse refractory material such as silica. Among the procedures ofinvesting the wax pattern which may be used is that of providing the waxpattern with a base portion of wax or suitable heat-destructiblematerial and then placing the base on a removable base of a metal flask.The refractory mold material which may consist of a mixture ofconventional silica having an ethyl silicate binder is then poured aboutthe pattern. An example of an investement dry mix or grog which may beused is one comprising major proportions of a finely ground, dead burnedfire clay and silica flour and minor proportions of magnesium oxide andborax glass. The binder for this grog may include an aqueous solution ofcondensed ethyl silicate, ethyl alcohol and hydrochloric acid.Preferably to 2% of borax is added to the mix to provide the refractorymix with greater strength enabling it to withstand the vacuum process.When the mold body has solidified or set to a suflicient extent to beself-supporting, the base of the flask is removed and heat is applied tothe mold whereby the pattern material is melted and permitted to run outof the mold at the point where the base of the pattern rested on thebase of the flask, and to convert the refractory material of the moldinto a relatively strong, self-sustaining mass. Preferably the mold isfirst heated to about 1300 F. to melt and remove the wax patternmaterial and then heated rapidly to about a 1900 F firing temperature.The opening formed in the base of the mold serves as a sprue and gate ofthe mold.

While the present invention has been described by means of specificexamples, it will be understood that the scope of the invention is notto be limited thereby except as defined in the following claims.

I claim:

1. A cast turbine blade comprising an alloy taken from the classconsisting of nickel and cobalt base alloys containing both chromium andcarbon having the leading and trailing edges thereof each consisting ofsubstantially a single crystal.

2. A cast turbine blade comprising a nickel base alloy containing bothchromium and carbon and having the leading and trailing edges thereofeach consisting of substantially a single crystal.

3. A cast turbine blade comprising an alloy consisting of 0.10% to 0.20%carbon, up to 0.25% manganese, up to 0.75% silicon, 13% to 17% chromium,4% to 6% molybdenum, 1.5% to 3% titanium, 2% to 4% aluminum, .01% to0.1% boron, 3.5% to 6% iron, and the balance nickel, having at least theleading and trailing edges thereof each consisting of substantially asingle crystal.

Vacuum Metallurgy, pages to 105. Papers presented at the VacuumMetallurgy Symposium of the Electrothermics and Metallurgy Division ofthe Electrochemical Society, October 6 and 7, 1954, at Boston,Massachusetts.

1. A CAST TURBINE BLADE COMPRISING AN ALLOY TAKEN FROM THE CLASSCONSISTING OF NICKEL AND COBALT BASE ALLOYS CONTAINING BOTH CHROMIUM ANDCARBON HAVING THE LEADING AND TRAILING EDGES THEREOF EACH CONSISTING OFSUBSTANTIALLY A SINGLE CRYSTAL.